In the field of medical imaging, diagnostic information from medical equipment is often output for viewing on a photographic element. Typically, the diagnostic information is recorded on the photographic element by exposing the element to radiation emitted by a radiation source such as a semiconductor laser diode. Other fields, such as graphic arts, also use photographic elements that are sensitized to imaging by a laser source. Typical laser sources include argon ion lasers, helium neon lasers as well as semiconductor laser diodes.
In laser imaging applications, the photographic element is exposed along one dimension by a laser beam that is modulated according to image data. The image data is a sequence of digital image values. Image processing electronics with the imaging system process the image data values to generate a sequence of digital laser drive values which control the intensity of the laser beam.
The modulated laser beam is incident upon a rotating, multi-faceted polygon which reflects the modulated laser beam across the photographic element in a raster pattern, thereby forming a plurality of scan lines of individual spots, known as pixels. The laser beam is modulated with the data at a fixed rate defined by a pixel clock. Typically, the polygon is mounted on an axis substantially orthogonal to the modulated laser beam and is spun by a motor. A complete image is formed on the photographic element by incrementally scanning the entire photographic element.
In order to achieve desirable image quality, such as image sharpness, the length of adjacent scan lines must be substantially equal. In high resolution imaging applications, such as medical imaging, the scan line length must remain constant to within a fraction of a pixel. Errors in scan line length may be caused by mechanical irregularities between individual polygon facets such as variations in "facet height", which is defined as the radial difference of each reflective surface from the rotational axis of the polygon. In order to minimize errors caused by variations in facet height, conventional polygon motor assemblies were constructed within tight mechanical tolerances. For this reason, conventional scanning assemblies are expensive to manufacture.
Other techniques have been developed for correcting facet height errors. For example, one technique generates the pixel clock via a voltage controlled oscillator (VCO) and adjusts the frequency of the VCO for each facet of the polygon in order to maintain a constant scan line length. The facet height error is calculated by measuring a time interval starting when the laser has been modulated with all of the image data and ending when an end-of-scan is received. By adjusting a reference voltage for the VCO, the frequency output of the VCO is controlled such that the proper number of pixels are formed between the start-of-scan signal and end-of-scan signal for each scan line. This technique, however, requires expensive circuitry. Furthermore, because the pixel clock itself is typically used to measure the facet height error, the accuracy of this technique is plus or minus one pixel clock per scan line which may be inadequate for high resolution imaging applications.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a scanning assembly that does not require such restrictive manufacturing techniques, is more tolerant to mechanical variations and compensates for facet height error with a high degree of accuracy.